One device for maintaining a fixed distance between a measuring device and an object to be measured, is as shown in FIG. 1. In the figure, reference character "a" designates a laser light source, such as an He - Ne laser; "b", a paired light receiving means; and "c", the surface of an object under measurement.
Light emitted from the light source "a" is incident on the surface "c" to be measured and reflected by the surface. The reflected light hits paired elements b.sub.1 and b.sub.2 of the light receiving means "b". When the surface "c" is positioned as indicated by a solid line in the figure, an output signal of the element b.sub.1 is large. When it is positioned as indicated by a two-dotted chain line in the figure, an output signal of the element b.sub.2 is large. A vertical distance between the light receiving means "b" and the surface "c" can be known from the output signals of the paired elements b.sub.1 and b.sub.2.
In the prior art, the light source "a" must be slanted with respect to the surface "c" of the object under measurement, in order that the reflected light is directed to the light receiving means "b". To increase a measurement accuracy, an angle .theta. of the light emitted from the light source should be preferably increased. To this end, a distance between the light source "a" and the light receiving means "b" must be increased. The light receiving means "b" consists of two elements b.sub.1 and b.sub.2, and hence tends to be bulky. Due to the above two factors, the position detector necessarily becomes bulky. The inclination of the surface "c" frequently causes a great measurement error.
A known device to correct the inclination of the surface to be measured is as shown in FIG. 2. Light receiving elements b.sub.3 and b.sub.4 have the same size, and are disposed equidistant from a light source "a". To correct the inclination of the surface "c", the surface is moved so as to zero a difference between the output signals of the elements b.sub.3 and b.sub.4.
However, this device is unable to measure the distance between the light receiving elements and the surface to be measured. Thus, neither of the conventional devices is able simultaneously to correct the distance between the light receiving elements and the surface to be measured, and the inclination of the surface.